Various personnel may be required to operate in conditions where harmful gases may be present in the atmosphere, which gases could cause respiratory failure. In view of the nature of the environment in which these people may be required to work they are customarily provided with gas masks provided with filters capable of filtering out the harmful gases. There is a requirement that a portable ventilator/resuscitator be provided which is capable for use with such personnel who are suffering respiratory failure and who are either wearing a gas mask of the type described above, or, in the alternative, who are not wearing such a mask. Thus, the portable ventilator/resuscitator must be provided with a mask assembly of the type which is capable of use with a patient wearing a gas mask, or, alternatively, with a patient who is not wearing a gas mask.